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The term diffraction can be defined as any deviation of rays from rectilinear paths that cannot be interpreted as either a reflection or a refraction. The classic description of the diffraction of a light ray is as follows: An aperture in an opaque screen is illuminated by a light source, and the light intensity is observed across a plane some distance behind the screen. The rectilinear theory of light propagation predicts that the shadow behind the screen should be well defined, with sharp borders. However, observation shows that the transition from light to shadow is gradual rather than abrupt. Given an appropriate light source, we can observe some striking results, such as the presence of light and dark fringes extending far into the geometric shadow of the screen. Such effects cannot be explained with a strict ray-based theory of light, which requires rectilinear propagation of light rays without reflection or refraction.